Switching mode power supplies are widely used in the power conversion field due to high efficiency. In the application, the switching frequency of the switching mode power supply is set to be relatively high to reduce the size of the passive devices (e.g., transformer, filter). However, high switching frequency would lead to high switching loss. Resonant technology is introduced to both reduce the switching loss and maintain the high switching frequency. Typically, resonant technology comprises an inductor and a capacitor coupled between a power stage and an output load of the switching mode power supply. The switching frequency is adjusted to regulate the output power.
FIG. 1 schematically shows graphs of the circuit gain Gain varying with the switching frequency f of the switching mode power supply using resonant technology, wherein f is the normalized frequency. Three graphs for the quality factor Q=1, Q=0.3 and Q=0.15 are illustrated. As shown in FIG. 1, the capacitor plays a dominant role inside the shaded portion, which is defined as capacitor mode; and the inductor plays a dominant role outside of the shaded portion, which is defined as inductive mode. When the switching frequency is at the left of the resonant frequency, the switching mode power supply would enter capacitor mode from the normal inductive mode if heavy load or short circuit suddenly happens (i.e., the quality factor Q increases suddenly). Under Inductive mode, power switches in the power stage can be controlled to be zero voltage switching (ZVS). On the contrary, capacitive mode is supposed to be avoided. Under capacitor mode, power switches in the power stage can not be controlled to be ZVS, which increases the switching loss. Furthermore, reverse recovery problem of the parasitic diode of the power switches would occur under capacitor mode, which would damage the power switches due to a shoot through between the high side power switch and the low side power switch.
Thus, there is a need to detect whether the switching mode power supply is under capacitor mode and to avoid undesired operation in the capacitor mode.